For many years, Internet Protocol (IP) technologies have supported packet-switched communications between computing devices through the use of packet forwarding devices such as routers and gateways. More specifically, blocks of information, generally referred to as “datagrams,” are routed between computing devices, where source and destination computing devices are identified by fixed length addresses. Such routing is connectionless in nature, in that the datagrams are forwarded between the packet forwarding devices on a hop-by-hop basis using the destination address in the datagram.
Mobility is a very important feature in wireless local area network (WLAN) solutions. When a mobile wireless (computing) device moves from one point of attachment to another, its IP address may have to be changed, especially when the later point of attachment is on a different network. IP address changes can cause some disruptions to the on-going IP applications. To avoid such disruptions, many technologies have been developed. These solutions can be divided into three major categories.
The first category relies on a mobile wireless device somehow detecting that it is now located on a new IP subnet, and as a result, automatically releases and renews it's IP address (via Dynamic Host Configuration Protocol “DHCP”). As noted, this IP address change can be disruptive to on-going IP applications. Hence, automatic IP address renewal is an ineffective solution in many instances.
The second category is a client-based solution such as “Mobile IP.” Mobile IP is an emerging “Layer 3” type protocol that allows a mobile wireless device to establish a wireless connection to an IP network. In general, Mobile IP is designed so that a mobile wireless device can acquire a new IP address, but still maintain its old IP address. This is accomplished through creation of mobility tunnels that will route datagrams from a prior (home) IP subnet to a new IP subnet and vice versa.
This tunneling is accomplished by implementing a Home Agent that maybe implemented on a router as part of the home IP subnet, which is responsible for maintaining current location information for the mobile wireless device and delivering datagrams to the mobile wireless device when roaming. When away from its home IP subnet, the router on the visitor IP subnet provides a “care-of address” for the mobile wireless device. A care-of address is a termination point of a mobility tunnel that enables information to be forwarded to the mobile wireless device while it is away from its home IP subnet.
In summary, this tunnel technology enables the routing of datagrams from the old IP subnet to the new IP subnet and vice versa. The main disadvantage of this approach is that it requires the implementation of Mobile IP within each mobile wireless device and ubiquitous support of Mobile IP in the infrastructure LAN devices.
The third category is an infrastructure or back-end based solution. In this solution, the mobile wireless device does not need to change its IP address. The back-end devices, such as access points “APs” and/or WLAN switches for example, will automatically tunnel traffic between the old attached network and the new attached network as well as related signaling processes. Since the IP address of the mobile wireless device does not change, the mobile wireless device maintains its point-of-presence in the network as being in the old IP subnet.
The problem is that the back-end infrastructure based solutions (like those described in the third category) requires extra resources on the back-end devices. Many implementations will try to close (or tear down) these back-end mobility tunnels when the back-end devices identify that these tunnels are not needed any more, namely the mobile wireless device has obtained a new IP address on the new IP subnet and no longer needs traffic to be tunneled to it from the old IP subnet. Without an acceptable housekeeping process, the number of open (and unused) tunnels increases and accumulates over time, consuming more and more network resources.
Previously, one solution for mobility tunnel closure involved timeouts due to lack of traffic. But, this is troublesome for devices with low duty cycle traffic patterns.
Another solution for mobility tunnel closure involved the back-end devices being adapted to sniff and detect client DHCP Discovery messages. When back-end devices, such as an AP or WLAN switch for example, detected the DHCP Discovery message sent from a mobile wireless device, those back-end devices assumed that the mobile wireless device did not need its old IP address. So, the back-end device closed any mobility tunnels that had been created for this mobile wireless device. This technique, however, is quite unreliable because there are certain situations where mobile wireless devices may send DHCP Discovery messages when it is still using the old IP address.